I'm having the same exact issue with my Streaming Stick+. And when I first bought the Stick+ my Google Assistant YouTube voice commands worked as well. And what's even more strange is that I can tell Google, "Launch YouTube on Living Room Roku" and it does... but from there, "Pause", "Stop", "Rewind" etc, returns the error with blank quotation marks. I'm totally lost as to what's going on...
Some other’s have posted about this exact same issue in another thread (myself included), see here: https://community.roku.com/t5/Discussions/Google-Home-YouTube-voice-commands-with-Roku-TV/m-p/597694
The latest info is that it appears that YouTube doesn’t support voice commands (e.g. try it also with the mic on your remote, if you have one). I suppose that’s how Roku works with assistant, but relaying the actual command from Google Assistant into the voice input in the app in Roku via their API’s? 🤔